It is widely conceded that bacteria, especially gram negative anaerobes, play an important role in the initiation and perpetuation of periodontal diseases. The accumulation of bacteria (plaque) near the gingiva will ultimately cause gingivitis, and developing periodontal lesions (pockets) are accompanied, if not caused by, accumulation of a distinctly anaerobic flora. Although considerable new information has derived from examination and classification of the pocket flora, there are many taxonomic problems remaining. Among these are the inability to classify many organisms and the realization that many oral isolates either do not conform precisely to established species descriptions, or that isolates which appear to be similar in many characteristics prove to be heterogeneous when examined in more detail. An example is the growing realization that Bacteroides melaninogenicus is a group of three organisms, one of which is now a separate species, and that there are additional heterogeneities within these subgroups. The work proposed here will clarify the taxonomic relationships among organisms isolated from periodontal pockets and determine if they are related to each other and to recognized species. These microbes will be compared by determining the similiarities of their deoxyribonucleic acid (DNA) base sequences by means of DNA-DNA hybridization. When organisms have been classified by genotype, their characteristics will be reviewed to identify phenotypic traits by which the genotype can be identified. Distinct genetic types will be considered species. The occurrence of these species in periodontal lesions will be determined by sampling pockets in several patients. We propose to study the relationships among B. asaccharolyticus strains isolated from monkeys, dogs, and man. We will analyze the DNA of Actinobacillus actinomycetemcomitans and we will initiate studies on oral spirochetes.